U.S. patent application number 10/965482 was filed with the patent office on 2005-05-05 for method for measuring the filling level of a liquid in a cavity having a sub-mm wide opening.
Invention is credited to Neumann, Rudolf, Stoecker, Stefan.
Application Number | 20050094148 10/965482 |
Document ID | / |
Family ID | 34529924 |
Filed Date | 2005-05-05 |
United States Patent
Application |
20050094148 |
Kind Code |
A1 |
Neumann, Rudolf ; et
al. |
May 5, 2005 |
Method for measuring the filling level of a liquid in a cavity
having a sub-mm wide opening
Abstract
The invention relates to a method to optically measure the
filling level of a liquid in a cavity having a sub-mm wide opening
with the aid of a measuring sensor based on chromatic coding, the
measuring sensor delivering a distance value and an intensity value
as its output signal. The method is characterized by the steps:
Recording a distance profile and an intensity profile of the
surface of the liquid by moving the measuring sensor along the
opening of the cavity holding the liquid, the movement being made
essentially parallel to the surface of the fluid along a
measurement segment x, and determining the filling level by means
of a combined analysis of the recorded distance profile and
intensity profile.
Inventors: |
Neumann, Rudolf;
(Spaichingen, DE) ; Stoecker, Stefan; (Bergisch
Gladbach, DE) |
Correspondence
Address: |
NORMAN H. ZIVIN
Cooper & Dunham LLP
1185 Avenue of the Americas
New York
NY
10036
US
|
Family ID: |
34529924 |
Appl. No.: |
10/965482 |
Filed: |
October 14, 2004 |
Current U.S.
Class: |
356/436 ;
250/577 |
Current CPC
Class: |
G01F 23/292
20130101 |
Class at
Publication: |
356/436 ;
250/577 |
International
Class: |
G01F 023/30; G01N
021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 30, 2003 |
DE |
103 50 716.7 |
Claims
1. A method to optically measure the filling level of a liquid in a
cavity having a sub-mm wide opening with the aid of a measuring
sensor based on chromatic coding, the measuring sensor delivering a
distance value and an intensity value as its output signal,
characterized by the steps, recording a distance profile and an
intensity profile of the surface of the liquid by moving the
measuring sensor along the opening of the cavity holding the
liquid, the movement being made essentially parallel to the surface
of the fluid along a measurement segment x, and determining the
filling level by means of a combined analysis of the recorded
distance profile and intensity profile.
2. A method according to claim 1, characterized in that the
distance value for which the intensity profile shows its maximum is
ascertained to be the filling value.
3. A method according to claim 1 or 2, characterized in that the
measured distance value is compared to the distance of a reference
surface and as a result, the level of the fluid is determined.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a method for measuring the filling
level of a liquid in a cavity having a sub-mm wide opening. In
general, the invention relates to the field of non-contact optical
distance measuring technology.
OUTLINE OF THE PRIOR ART
[0002] In a hydrodynamic fluid bearing of the kind employed in
spindle motors for example, the bearing gap has to be filled with a
lubricating fluid, such as oil, before the motor is put into
operation for the first time. Here, the filling level of the
lubricant in the bearing gap is critical and goes to determine,
among other factors, the useful life of the bearing. The bearing
gap and also the opening for filling in and measuring the filling
level of the lubricant, are very small and lie in the sub-mm
range.
[0003] In a preferred embodiment of such a bearing, at one of its
ends, the bearing tube has a tapered area taking the form of a
conical or cylindrical counterbore, whereas the opposite end of the
bearing is hermetically sealed. The countersink in the bearing tube
goes to form a concentric tapered area between the inner surface of
the tube and the outer surface of the shaft that widens towards the
top end and is proportionately filled with bearing oil. The oil
covers the surfaces of the tube and shaft as a result of which a
meniscus, having a concave surface, is formed on the contact
surface to the air. The bearing oil in the tapered area acts as a
lubricant reservoir from which evaporated bearing oil is replaced.
The area above the meniscus between the inner sleeve surface of the
cone and the outer sleeve surface of the shaft acts as an
equalizing volume into which the bearing oil can rise when its
temperature-dependent volume increases as the temperature rises and
thus causes the fluid level to increase. The cohesive forces acting
in the fluid of the lubricant, supported by the capillary forces in
the bearing gap, prevent liquid bearing oil from escaping from the
bearing and penetrating into the clean room area.
[0004] It is clear that measuring the filling level in the
lubricant reservoir of a fluid bearing is not a trivial matter and
is made more difficult by the formation of a meniscus. Special
measuring techniques are therefore necessary to measure the filling
level. To date, the filling level of the lubricant has been
measured using a geometrical optics method. A well-known measuring
technique is based on directing parallel light to a reference
surface and to the surface of the lubricant. The distance between
the reflection maxima allows the filling level of the lubricant in
the reservoir to be determined.
[0005] This measuring technique, however, is only suitable for
sufficiently large cavity openings. This method does not work in
the case of a fluid bearing having very small bearing gaps.
Furthermore, a numerical distance value is not provided.
SUMMARY OF THE INVENTION
[0006] The object of the invention is to provide a method for
measuring the filling level of a liquid in a cavity having a sub-mm
wide opening which provides exact filling level values even for an
opening having the smallest dimensions.
[0007] This object has been achieved in accordance with the
invention by the characteristics outlined in patent claim 1.
[0008] Beneficial embodiments of the invention are outlined in the
dependent claims.
[0009] The invention is based on a method belonging to the field of
non-contact distance measuring technology and, in particular, on an
optical measuring technique based on a wavelength-dependent, that
is chromatic, coding of the area that is to be measured. This
chromatic measuring technique in itself is known.
[0010] This measuring technique has been put into practice, for
example, in the optical micrometer measuring sensor FRT-CWL made by
FRT GmbH, which is a chromatic white light sensor. The sensor is
well suited for measuring contour, roughness and topography. To
determine the distance to a sample surface, this surface is
illuminated with a focused white light which is fed from a light
source via a fiber optic cable to the sample. Passive optics having
a large chromatic aberration split the light vertically in focus
points of various colors, and thus height, and is reproduced on the
sample. The chromatic aberration results in a strong
wavelength-dependent focal length for this reproduction. If an
optical surface is now located within this focal range, only the
wavelength whose focus lies on the surface is sharply reproduced.
Conversely, only the reflection of this wavelength is again
reproduced sharply at the end of the optic fiber and coupled into
the fiber (confocal principle). Here, it is not important if the
surface reflects in a diffusely scattered or mirror-like way. The
light reflected from the sample surface is fed through the same
optics and the optic fiber cable to a spectrometer. From the color
of the light ascertained there, the position of the focus point can
be determined using a calibration table and thus the position of
the sample surface. Since the sensor works without using an active
control, very rapid measurements on structured surfaces are made
possible.
[0011] The chromatic white light sensor delivers a distance value
and an intensity value as its output signal. In accordance with the
invention, the sensor is moved along the opening of a cavity
holding the liquid essentially parallel to the surface of the
liquid. While the sensor is being moved, distance values and
intensity values are continuously measured, producing a distance
profile and an intensity profile of the surface of the liquid. The
actual filling level of the liquid can be determined from a
combined analysis of the distance profile and the intensity
profile.
[0012] According to the invention, the distance value for which the
intensity profile shows its maximum is ascertained to be the
filling value. To determine an absolute filling level, the distance
value can be compared to the distance of a reference surface and as
a result, the level of the fluid determined.
[0013] The invention will now be explained in more detail on the
basis of an embodiment with reference to the drawings.
BRIEF DESCRIPTION OF THE INVENTION
[0014] FIG. 1 shows a section through a hydrodynamic bearing
arrangement as employed, for example, in spindle motors;
[0015] FIG. 2 shows the measurement arrangement on the basis of an
enlarged view of the bearing arrangement in the region of the
lubricant reservoir;
[0016] FIG. 3 shows the output signal delivered by the optical
sensor in the form of an intensity and distance profile.
DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
[0017] The embodiment describes a hydrodynamic bearing arrangement
as employed, for example, in a spindle motor.
[0018] In accordance with FIG. 1, the bearing arrangement comprises
a shaft 1 arranged freely rotatably in a bearing tube 2. One of the
surfaces of the shaft 1 and/or tube 2 that face each other, the
surface of the shaft 1 in the case illustrated, has cylindrical
zones engraved with groove patterns 3.
[0019] The tube 2 is provided at its bottom end with an annular
recess to accommodate a thrust plate 4. In the same way as the
shaft 1 rotates in the tube 2, the thrust plate 4, firmly connected
to the shaft, rotates in the recess. The lower opening of the tube
2 is hermetically sealed by a cover 5 which prevents air from
penetrating into the bearing arrangement.
[0020] A liquid lubricant, such as oil, is filled into the bearing
gap that is formed between the shaft/thrust plate and bearing
sleeve/cover. The groove pattern 3 mentioned above, causes a kind
of pumping action to be created when the shaft 1 rotates, leading
not only to a build-up of pressure but also to the lubricant being
distributed.
[0021] At its top end, the bearing tube 2 has a tapered area taking
the form, for example, of a conical counterbore which, together
with the outside diameter of the shaft 1, forms a reservoir 6 to
hold the lubricant. The reservoir 6 has its largest diameter at the
top end of the tube 2.
[0022] When the bearing arrangement has been completed, the
reservoir 6 can be filled with the lubricant 7 in accordance with
FIG. 2. The filling level of the lubricant 7 in the reservoir 6 can
now be optically checked using the lubricant meniscus 8 that is
built up in the reservoir 6 due to capillary action.
[0023] The measuring technique according to the invention will be
explained on the basis of FIGS. 2 and 3. A chromatic white light
sensor 9 is arranged in such a way that it looks perpendicularly
into the opening in the lubricant reservoir 6. The light beam 10
emitted by the sensor 9 hits the surface of the lubricant defined
by the meniscus 8. The light reflected from the surface 8 is fed to
a spectrometer of the measuring arrangement (not illustrated). From
the color of the light determined there, the position of the focus
point can be determined using a calibration table and thus the
distance d between the sensor and the surface of the lubricant 8.
During the measuring process, the sensor 9 is moved in a lateral
direction along a measurement segment x parallel to the opening in
the reservoir 6. The distance between the sensor 9 and the surface
of the lubricant 8 and the intensity of the light reflected from
the surface of the lubricant are constantly measured. This goes to
produce an intensity profile 11 and a distance profile 12 over the
measurement segment x, as shown by way of example in FIG. 3.
[0024] It can be clearly seen that the distance profile 12 does not
follow the concave course of the lubricant meniscus 8 as expected,
but falls steadily over the measurement segment x. The main reason
for this is the extraneous reflection of the light 10 emitted by
the sensor 9 off the sloping surface 13 of the bearing tube 2 that
forms the side wall of the reservoir 6. This means that the actual
level of the liquid that corresponds to the meniscus 8 minimum
cannot be determined from the distance profile alone. The distance
value d at which the surface of the meniscus 8 is at its lowest
point (minimum) has to be found.
[0025] According to the invention, the solution to this problem
lies in analyzing the intensity profile 11 in addition to the
distance profile. The intensity profile shows that the intensity of
the reflected light runs reciprocally to the curve of the meniscus
and is at its greatest when the light beam 10 is reflected exactly
perpendicularly back into the sensor 9 from the surface of the
meniscus. This point, indicated here as intensity maximum
I.sub.max, defines the lowest point of the meniscus. The lateral
position x.sub.Max associated with the intensity maximum, can now
be applied to the distance profile. The distance value d.sub.Oel
taken at position x.sub.max represents the sought after filling
level of the lubricant.
[0026] The absolute value of the filling level can be determined by
comparing the measured distance value d with a reference value, for
example, the distance between the sensor 9 and the top surface 14
of the bearing tube 2.
Identification Reference List
[0027] 1 Shaft
[0028] 2 Bearing tube
[0029] 3 Groove pattern
[0030] 4 Thrust plate
[0031] 5 Cover
[0032] 6 Reservoir
[0033] 7 Lubricant
[0034] 8 Meniscus
[0035] 9 Optical sensor
[0036] 10 Light beam
[0037] 11 intensity profile
[0038] 12 Distance profile
[0039] 13 Sloping surface
[0040] 14 Surface
* * * * *